I'd read online that it's possible to cure sodium silicate and sand with a microwave oven. I'm using 100 gram samples with 3% sodium silicate and it works fine....sand gets a bit hot though. By way of proof I tender this indistinct photo with two lumps of sand that could have been stuck together with anything.
After trying a larger test sample it looks like the heating from the microwaves is driving off some moisture and making the silicate and sand harden. When I heat it in a plastic cup the bottom region is slower to cure than the top surface where the moisture steams off.
Pure sodium silicate is a white powder, if it's made using the aqueous method then you're reacting caustic soda, sand and water to get the liquid form. Edit:Looks like I was guessing correctly: according to Wikipedia, heating liquid sodium silicate above 105 degrees C or 221 deg F will drive off water and make it go into "Glass phase" and solidify. Probably not going to be a practical method without heat resistant patterns.
I put cores in the oven and heat them to make them hard. My homemade SS doesn't set up very well with just CO2. Backing them makes them like cement and tough to chip out. I'm going to try less SS next time. If I ever get everything unpacked from our move back to Ohio.
Stop Nuking this out, and concentrate on casting... your sand needs to be clay free, and have less then 1/2 percent moisture. you mix 3 parts sodium silicate to 100 parts sand... so again where are you getting moisture in your CO2 sand... if it has moisture, you have another issue.. crumbly sand!!! V/r HT1
So heating is just another way of helping the sodium silicate to set properly, kind of makes sense as the chemistry rule of thumb is that every ten degrees C increase doubles the speed of a reaction. The thing that surprised me was that it's possible to make waterglass cure without CO2 at all and according to some online papers you end up with a stronger bond that way. I found this article about the topic: www.foundryworld.com/uploadfile/200912130267115.pdf The main gist of it is that you can use less sodium silicate at 1-2% in the sand when microwaving it and that 24 hours later it absorbs enough moisture for strength to drop to 20% (page 194 of article). Makes you wonder if you make a sand mould, bake it hard and dry, use it within an hour or so and then 24-48 hours later when it gets damp, it all falls apart and could possible be shaken out and reused by baking again. It looks like microwaving the cores is just another way to heat, it's kind of cool to get faster than Pepset speeds of around 30 seconds to go hard, but it may not be practical for anything other than small cores. More testing is required...
The first sample, which initially I was bouncing off the concrete floor intact is mostly crumbled after 16 hours in 80-90% humidity, except for the core 3/4" or so. I repacked it into the plastic cup and microwaved it and it went concrete hard again..... go figure... here's something that sets like concrete for as long as you can keep it dry and disintegrates after a day or so in a humid climate. Now the only question remains is does it remain reusable after getting molten metal poured in it. Sample on the left is 12 hours old and is actually noticeable wetter than when I made it. The sample on the right is the very first sample that was mostly crumbled into brown sugar consistency and freshly packed into the mould and re-microwaved. It's the local beach sand and has not been washed to remove salt either, just sun dried on a tarp and packed into a plastic bin. It has some mineral sands in it such as ilmenite and rutile.
I have used sodium silicate for both cores and entire molds, with good success, but it took me a while to figure out how to do it correctly. Myfordboy warned in one of his videos (I think it was him) to not over-gas sodium silicate bound sand. Somehow I missed that video, or that part of the video, so I put my cores in ziplock bags and filled them with CO2. Too much CO2 is the worst think you can do to a SS core or mold, and it will not have any strength at all. To correct the problem, I went from 3% SS up to perhaps 8% SS, and that made some cores that were like granite, and impossible to remove from a casting. I finally watched the video one more time and picked up on the "don't gas your cores more than 5 seconds". I did not believe it, but I had nothing to lose (sadly, I often find that I am of the mentality that more must be better in all things, and I end up learning the hard way. It does not seem plausible that "less can be more"). I dropped back to 3% SS, gassed for 5 seconds only, and the cores are very stable. I looked at some cores on the shelf in the shop yesterday that I made a year ago using the 3%/5 second rule, and they are still perfect and usable. I have always used clean very dry sand for making cores, so I can't comment what moisture may do to cores with SS. The sand I use comes with I think less than 1/2% moisture (or maybe less, I will have to check), and I think it is probably baked at the factory to get it that dry. I keep it stored in 5 gallon sealed plastic containers. And I have not tried the microwave trick either, but it would be worth a try. I generally make round cores using a piece of PVC pipe slit down one side, and I am not sure how well the PVC would hold up in the microwave. I also bought some SS that comes with a hardener last year, but have not used it yet. The self-hardening SS sand is especially useful for making entire molds from SS sand, since you don't have to try and gas thick sections of sand.
Pure PVC with no fillers is very stable in radio frequency fields, radio HAMs here use the orange electrical PVC as it has very little fillers unlike the plumbing PVC and one of their tests to see if it has dielectric losses is to give it a hit in the microwave along with a mug of water as a dummy load and see it it gets hot. The sand is going to get pretty hot however in contact with the sand. I've been zapping it enough to solidify it and then giving it a longer blast without the mould to really dry it.
Here is an interesting link to an article that suggests microwaving SS may be better than CO2 treatment. http://www.afe.polsl.pl/index.php/e...ter-glass-on-structure-of-bonding-bridges.pdf I am presently (tonight) molding a fairly large 2-3 pound core from SS and silica sand. I have been having a hard time gassing it as it is pretty thick and diffusing gas evenly and deeply seems to be a challenge. Looks like I have finally got one core that may work for proof of concept. Very interesting comments from Pat concerning over-gassing. I may be another guilty party :-( I want to get this worked out as I have a new pattern that depends on 3 largish cores per casting.
PatJ, can you tell me a little more about the self-hardening SS you are using. Where do you get it or how do you mix it. Does it use portland cement as a self-hardening agent?
I bought 5 gallons of Chembond 4905 SS from Clay Planet: http://shop.clay-planet.com/chembond-4905-no-bake-binder---5-gallon-bucket.aspx and a quart of Chembond Catalyst 210: http://shop.clay-planet.com/chembond-catalyst-210.aspx If you go to the pdf file for the Chembond Catalyst, you can get it in various speeds, but Clay Planet appears to only sell the 210, which has a pretty long strip time. http://www.ha-international.com/tds/321517.pdf The motorcycle racing guy from AA (Henise?) found some of the faster catalyst out in the San Fran area, but I don't have a supplier name. He may have had to buy 5 gallons of the 250 catalyst. I have used resin binder in the past, and it was a 3-part mix, with a resin, hardener, and catalyst, and you could easily vary the set and strip times by varying the amount of catalyst. For the Chembond SS, you can only vary the set and strip times by using a different catalyst number. I have been looking for 250, which has a strip time of 10-15 minutes, which means a set time shorter than that. For smaller molds, you can mix and ram them in perhaps 5 minutes (before the set time expires). For larger molds/cores, you may need more set time. The procedure is to mix the SS with the sand first for about 2 minutes, and then start a stopwatch as you mix in the catalyst/hardener. It takes 2 minutes of your set time to mix in the catalyst, so be sure to put that in your calculation. You must finish all the ramming before the set time, and you must pull the pattern at the strip time, else the pattern will be cemented permanently into the mold (if you are making molds using bound sand). The reason I have converted to SS is that it does not have the toxic fumes that the resin binders have, although the resin-bound sand is the cat's meow as far as what it can do with molding and how well it works with iron. The resin-binder directions mention that the sand has to be very dry (less than 0.25% moisture), must be clean, and must not contain clay or any other contaminate, in order for the binder to work as advertised at the set/strip times published. I have also heard that you can lightly flame cores and molds made from SS, and I have started doing that to drive out any residual moisture, and it seems to work. I use a standard plumber's propane torch. Don't get carried away with the torch or you will burn up your core/mold.
That's a good read: 10 to 100 times less energy to heat the sand mix compared to a conventional oven and 10-200 times faster to cure. The compression strengths were 12-26 times stronger than CO2 curing. Edit: I recycled some of the original disintegrating test pieces for the 2nd and 3rd time round in the microwave to set them and then transferred them to the conventional electric oven at 120 deg C or 248 deg F for half an hour to get them above the 105 deg C glass phase and they now appear to be mechanically stable after a few hours. Looks like the test pieces have to hit that temperature to cure properly. It doesn't appear any wetter than the sealed control at this stage but already it's holding together much better than all of the earlier pieces that would be losing the surface grains of sand by now.
The standard SS I have been buying by the gallon from Budget Castings has never had a shelf life problem. It seems to last indefinitely. I assume that the 5 gal of SS from Clay Planet is also stable, but that is just a guess since I have not used it yet. The resin-binder hardener did have a shelf life, and it would crystallize when exposed to air. I have not used the Clay Planet SS catalyst, so I don't know about its shelf life, but it is relatively inexpensive, and readily available (unlike resin binder), so even if it does have a shelf life, I don't think it will be a problem.
Shelf life has got to be the one major weakness of some of the fast no bake resin binders. I'm no chemist but there's got to be a cross linker additive and while they give speed, extra strength and heat resistance to a resin the raw cross linker breaks down fairly quickly from light and moisture. Six months is about the limit and it's beginning to slow down. The latest test sample which was microwaved to solidify it and then baked in an electric oven at 120 deg C to fully cure it has held it's shape for 15 hours in rainy weather. It's still holding it's shape but is losing sand if you rub it hard with your fingers, about on a par with cured no-bake resin strength. The identical sample in a plastic bag is perfectly fine.
